Mac layer communication for parallel random access procedures of dual connectivity

ABSTRACT

The present invention relates to MAC layer communication for parallel random access procedures of dual connectivity. In this scheme, a first MAC entity of a user equipment informs the initiation of the first random access procedure to a second MAC entity of the UE; and informs the completion of the first random access procedure to the second MAC entity, when the first random access procedure is finished

TECHNICAL FIELD

The present invention relates to a random access procedure in a wirelesscommunication system and, more particularly, to methods for MAC layercommunication for parallel random access procedures of dual connectivityand devices therefor.

BACKGROUND ART

As an example of a mobile communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project Long TermEvolution (hereinafter, referred to as LTE) communication system isdescribed in brief.

FIG. 1 is a view schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system. An Evolved UniversalMobile Telecommunications System (E-UMTS) is an advanced version of aconventional Universal Mobile Telecommunications System (UMTS) and basicstandardization thereof is currently underway in the 3GPP. E-UMTS may begenerally referred to as a Long Term Evolution (LTE) system. For detailsof the technical specifications of the UMTS and E-UMTS, reference can bemade to Release 7 and Release 8 of “3rd Generation Partnership Project;Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs(eNBs), and an Access Gateway (AG) which is located at an end of thenetwork (E-UTRAN) and connected to an external network. The eNBs maysimultaneously transmit multiple data streams for a broadcast service, amulticast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in oneof bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides adownlink (DL) or uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be set to provide differentbandwidths. The eNB controls data transmission or reception to and froma plurality of UEs. The eNB transmits DL scheduling information of DLdata to a corresponding UE so as to inform the UE of a time/frequencydomain in which the DL data is supposed to be transmitted, coding, adata size, and hybrid automatic repeat and request (HARQ)-relatedinformation. In addition, the eNB transmits UL scheduling information ofUL data to a corresponding UE so as to inform the UE of a time/frequencydomain which may be used by the UE, coding, a data size, andHARQ-related information. An interface for transmitting user traffic orcontrol traffic may be used between eNBs. A core network (CN) mayinclude the AG and a network node or the like for user registration ofUEs. The AG manages the mobility of a UE on a tracking area (TA) basis.One TA includes a plurality of cells.

Although wireless communication technology has been developed to LTEbased on wideband code division multiple access (WCDMA), the demands andexpectations of users and service providers are on the rise. Inaddition, considering other radio access technologies under development,new technological evolution is required to secure high competitivenessin the future. Decrease in cost per bit, increase in serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, and the likeare required.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies inthe conventional mobile communication system. The technical problemssolved by the present invention are not limited to the above technicalproblems and those skilled in the art may understand other technicalproblems from the following description.

Technical Solution

To achieve the object of the present invention, in one aspect, a methodof communicating with a network at a user equipment (UE) in a wirelesscommunication system is provided. This method comprises: initiating afirst random access procedure of a first MAC entity of the UE; informingthe initiation of the first random access procedure to a second MACentity of the UE; and informing a completion of the first random accessprocedure to the second MAC entity by the first MAC entity, when thefirst random access procedure is finished.

In another aspect of the present invention, a user equipment (UE)communicating with a network in a wireless communication system isprovided. This UE comprises: a transceiver configured to transmit andreceive signals to and from a first base station and a second basestation; and a processor connected to the transceiver and comprising afirst MAC entity and a second MAC entity, wherein the first MAC entityconfigured to initiate a first random access procedure of a first MACentity of the UE, wherein the processor is configured to control thefirst MAC entity to inform the initiation of the first random accessprocedure to the second MAC entity of the UE; and to inform a completionof the first random access procedure to the second MAC entity, when thefirst random access procedure is finished.

The second MAC entity of the UE may not inform the first MAC entity ofan initiation or a completion of a second random access procedure of thesecond MAC entity.

The second MAC entity of the UE may stop ongoing second random accessprocedure when the second MAC entity is informed the initiation of thefirst random access procedure from the first MAC entity.

The second MAC entity of the UE may restart a second random accessprocedure when the second MAC entity is informed the completion of thefirst random access procedure from the first MAC entity and there is astopped second random access procedure. Here, the second MAC entity ofthe UE may restart the second random access procedure as a contentionbased random access procedure, even though the stopped second randomaccess procedure was a non contention based random access procedure.

The second MAC entity of the UE may ignore a command from a second basestation to initiate a second random access procedure, if the second MACentity is informed the initiation of the first random access procedurefrom MAC entity and did not be informed the completion of the firstrandom access procedure from the first MAC entity.

The first MAC entity may be responsible for transferring first type dataand the second MAC entity may be responsible for transferring secondtype data, where the second type data is delay tolerable comparing tothe first type data.

The first MAC entity may be responsible for transferring data to a firstbase station and the second MAC entity may be responsible fortransferring data to a second base station, where a first service areaof the first base station is bigger than a second service area of thesecond base station.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to the present invention, parallel random access procedure toimplement the dual connectivity of the UE would efficiently operate.Specifically, the UE can perform the random access procedure for SeNBwith minimum interference with the random access procedure for MeNB.

It will he appreciated by persons skilled in the art that that theeffects achieved by the present invention are not limited to what hasbeen particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system;

FIG. 2 is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS).

FIG. 3 is a block diagram depicting architecture of a typical E-UTRANand a typical EPC;

FIG. 4 is a diagram showing a control plane and a user plane of a radiointerface protocol between a UE and an E-UTRAN based on a 3GPP radioaccess network standard;

FIG. 5 is a view showing an example of a physical channel structure usedin an E-UMTS system;

FIG. 6 is a diagram for carrier aggregation;

FIG. 7 is a diagram illustrating an operation procedure of a userequipment and a base station during a non-contention based random accessprocedure;

FIG. 8 is a diagram illustrating an operation procedure of a userequipment and a base station during a contention based random accessprocedure;

FIG. 9 is a diagram for explaining dual connectivity of the UE accordingto one aspect of the present invention;

FIG. 10 is a diagram showing an exemplary architecture for supportingdual connectivity;

FIG. 11 is a diagram for explanation on a preferred embodiment of thepresent invention;

FIG. 12 is a diagram of another example of the present invention;

FIG. 13 is a diagram for explaining the procedure according to currentLTE standard considering dual connectivity;

FIG. 14 is a diagram for explaining one example of the presentinvention;

FIGS. 15 and 16 are detailed examples for the scheme explained withregards to FIG. 14; and

FIG. 17 is a block diagram of a communication apparatus according to anembodiment of the present invention.

BEST MODE

Universal mobile telecommunications system (UMTS) is a 3rd Generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). The long-term evolution (LTE) of UMTS is under discussion by the3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LIE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3GPP LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

Hereinafter, structures, operations, and other features of the presentinvention will be readily understood from the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Embodiments described later are examples in which technicalfeatures of the present invention are applied to a 3GPP system.

Although the embodiments of the present invention are described using along term evolution (LTE) system and a LTE-advanced (LTE-A) system inthe present specification, they are purely exemplary. Therefore, theembodiments of the present invention are applicable to any othercommunication system corresponding to the above definition. In addition,although the embodiments of the present invention are described based ona frequency division duplex (FDD) scheme in the present specification,the embodiments of the present invention may be easily modified andapplied to a half-duplex FDD (H-FDD) scheme or a time division duplex(TDD) scheme.

FIG. 2 is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS). The E-UMTS may bealso referred to as an LTE system. The communication network is widelydeployed to provide a variety of communication services such as voice(VoIP) through IMS and packet data.

As illustrated in FIG. 2, the E-UMTS network includes an evolved UMTSterrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC)and one or more user equipment. The E-UTRAN may include one or moreevolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 maybe located in one cell. One or more E-UTRAN mobility management entity(MME)/system architecture evolution (SAE) gateways 30 may be positionedat the end of the network and connected to an external network.

As used herein, “downlink” refers to communication from eNodeB 20 to UE10, and “uplink” refers to communication from the UE to an eNodeB. UE 10refers to communication equipment carried by a user and may be alsoreferred to as a mobile station (MS), a user terminal (UT), a subscriberstation (SS) or a wireless device.

FIG. 3 is a block diagram depicting architecture of a typical E-UTRANand a typical EPC.

As illustrated in FIG. 3, an eNodeB 20 provides end points of a userplane and a control plane to the UE 10. MME/SAE gateway 30 provides anend point of a session and mobility management function for UE 10. TheeNodeB and MME/SAE gateway may be connected via an S1 interface.

The eNodeB 20 is generally a fixed station that communicates with a UE10, and may also he referred to as a base station (BS) or an accesspoint. One eNodeB 20 may be deployed per cell. An interface fortransmitting user traffic or control traffic may be used between eNodeBs20.

The MME provides various functions including NAS signaling to eNodeBs20, NAS signaling security, AS Security control, Inter CN node signalingfor mobility between 3GPP access networks, Idle mode UE Reachability(including control and execution of paging retransmission), TrackingArea list management (for UE in idle and active mode), PDN GW andServing GW selection, MME selection for handovers with MME change, SGSNselection for handovers to 2G or 3G 3GPP access networks, Roaming,Authentication, Bearer management functions including dedicated bearerestablishment, Support for PWS (which includes ETWS and CMAS) messagetransmission. The SAE gateway host provides assorted functions includingPer-user based packet filtering (by e.g. deep packet inspection), LawfulInterception, UE IP address allocation, Transport level packet markingin the downlink, UL and DL service level charging, gating and rateenforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAEgateway 30 will be referred to herein simply as a “gateway,” but it isunderstood that this entity includes both an MME and an SAE gateway.

A plurality of nodes may be connected between eNodeB 20 and gateway 30via the S1 interface. The eNodeBs 20 may be connected to each other viaan X2 interface and neighboring eNodeBs may have a meshed networkstructure that has the X2 interface.

As illustrated, eNodeB 20 may perform functions of selection for gateway30, routing toward the gateway during a Radio Resource Control (RRC)activation, scheduling and transmitting of paging messages, schedulingand transmitting of Broadcast Channel (BCCH) information, dynamicallocation of resources to UEs 10 in both uplink and downlink,configuration and provisioning of eNodeB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE-IDLE state management,ciphering of the user plane, System Architecture Evolution (SAE) bearercontrol, and ciphering and integrity protection of Non-Access Stratum(NAS) signaling.

The EPC includes a mobility management entity (MME), a serving-gateway(S-GW), and a packet data network-gateway (PDN-GW). The MME hasinformation about connections and capabilities of UEs, mainly for use inmanaging the mobility of the UEs. The S-GW is a gateway having theE-UTRAN as an end point, and the PDN-GW is a gateway having a packetdata network (PDN) as an end point.

FIG. 4 is a diagram showing a control plane and a user plane of a radiointerface protocol between a UE and an E-UTRAN based on a 3GPP radioaccess network standard.

The control plane refers to a path used for transmitting controlmessages used for managing a call between the UE and the E-UTRAN. Theuser plane refers to a path used for transmitting data generated in anapplication layer, e.g., voice data or Internet packet data.

A physical (PHY) layer of a first layer provides an information transferservice to a higher layer using a physical channel. The PHY layer isconnected to a medium access control (MAC) layer located on the higherlayer via a transport channel. Data is transported between the MAC layerand the PHY layer via the transport channel. Data is transported betweena physical layer of a transmitting side and a physical layer of areceiving side via physical channels. The physical channels use time andfrequency as radio resources. In detail, the physical channel ismodulated using an orthogonal frequency division multiple access (OFDMA)scheme in downlink and is modulated using a single carrier frequencydivision multiple access (SC-FDMA) scheme in uplink.

The MAC layer of a second layer provides a service to a radio linkcontrol (RLC) layer of a higher layer via a logical channel. The RLClayer of the second layer supports reliable data transmission. Afunction of the RLC layer may be implemented by a functional block ofthe MAC layer. A packet data convergence protocol (PDCP) layer of thesecond layer performs a header compression function to reduceunnecessary control information for efficient transmission of anInternet protocol (IP) packet such as an IP version 4 (IPv4) packet oran IP version 6 (IPv6) packet in a radio interface having a relativelysmall bandwidth.

A radio resource control (RRC) layer located at the bottom of a thirdlayer is defined only in the control plane. The RRC layer controlslogical channels, transport channels, and physical channels in relationto configuration, re-configuration, and release of radio bearers (RBs).An RB refers to a service that the second layer provides for datatransmission between the UE and the E-UTRAN. To this end, the RRC layerof the UE and the RRC layer of the E-UTRAN exchange RRC messages witheach other.

One cell of the eNB is set to operate in one of bandwidths such as 1.25,2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplinktransmission service to a plurality of UEs in the bandwidth. Differentcells may be set to provide different bandwidths.

Downlink transport channels for transmission of data from the E-UTRAN tothe UE include a broadcast channel (BCH) for transmission of systeminformation, a paging channel (PCH) for transmission of paging messages,and a downlink shared channel (SCH) for transmission of user traffic orcontrol messages. Traffic or control messages of a downlink multicast orbroadcast service may be transmitted through the downlink SCH and mayalso be transmitted through a separate downlink multicast channel (MCH).

Uplink transport channels for transmission of data from the UE to theE-UTRAN include a random access channel (RACH) for transmission ofinitial control messages and an uplink SCH for transmission of usertraffic or control messages. Logical channels that are defined above thetransport channels and mapped to the transport channels include abroadcast control channel (BCCH), a paging control channel (PCCH), acommon control channel (CCCH), a multicast control channel (MCCH), and amulticast traffic channel (MTCH).

FIG. 5 is a view showing an example of a physical channel structure usedin an E-UMTS system.

A physical channel includes several subframes on a time axis and severalsubcarriers on a frequency axis. Here, one subframe includes a pluralityof symbols on the time axis. One subframe includes a plurality ofresource blocks and one resource block includes a plurality of symbolsand a plurality of subcarriers. In addition, each subframe may usecertain subcarriers of certain symbols (e.g., a first symbol) of asubframe for a physical downlink control channel (PDCCH), that is, anL1/L2 control channel. In FIG. 5, an L1/L2 control informationtransmission area (PDCCH) and a data area (PDSCH) are shown. In oneembodiment, a radio frame of 10 ms is used and one radio frame includes10 subframes. In addition, one subframe includes two consecutive slots.The length of one slot may be 0.5 ms. In addition, one subframe includesa plurality of OFDM symbols and a portion (e.g., a first symbol) of theplurality of OFDM symbols may be used for transmitting the L1/L2 controlinformation. A transmission time interval (TTI) which is a unit time fortransmitting data is 1 ms.

A base station and a UE mostly transmit/receive data via a PDSCH, whichis a physical channel, using a DL-SCH which is a transmission channel,except a certain control signal or certain service data. Informationindicating to which UE (one or a plurality of UEs) PDSCH data istransmitted and how the UE receive and decode PDSCH data is transmittedin a state of being included in the PDCCH.

For example, in one embodiment, a certain PDCCH is CRC-masked with aradio network temporary identity (RNTI) “A” and information about datais transmitted using a radio resource “B” (e.g., a frequency location)and transmission format information “C” (e.g., a transmission blocksize, modulation, coding information or the like) via a certainsubframe. Then, one or more UEs located in a cell monitor the PDCCHusing its RNTI information. And, a specific UE with RNTI “A” reads thePDCCH and then receive the PDSCH indicated by B and C in the PDCCHinformation.

FIG. 6 is a diagram for carrier aggregation.

Carrier aggregation technology for supporting wide bandwidth. Asmentioned in the foregoing description, it may be able to support systembandwidth up to maximum 100 MHz in a manner of bundling maximum 5carriers (component carriers: CCs) of bandwidth unit (e.g., 20 MHz)defined in a legacy wireless communication system (e.g., LTE system) bycarrier aggregation. Component carriers used for carrier aggregation maybe equal to or different from each other in bandwidth size. And, each ofthe component carriers may have a different frequency band (or centerfrequency). The component carriers may exist on contiguous frequencybands. Yet, component carriers existing on non-contiguous frequencybands may be used for carrier aggregation as well. In the carrieraggregation technology, bandwidth sizes of uplink and downlink may beallocated symmetrically or asymmetrically.

Multiple carriers (component carriers) used for carrier aggregation maybe categorized into primary component carrier (PCC) and secondarycomponent carrier (SCC). The PCC may be called P-cell (primary cell) andthe SCC may be called. S-cell (secondary cell). The primary componentcarrier is the carrier used by a base station to exchange traffic andcontrol signaling with a user equipment. In this case, the controlsignaling may include addition of component carrier, setting for primarycomponent carrier, uplink (UL) grant, downlink (DL) assignment and thelike. Although a base station may be able to use a plurality ofcomponent carriers, a user equipment belonging to the corresponding basestation may be set to have one primary component carrier only. If a userequipment operates in a single carrier mode, the primary componentcarrier is used. Hence, in order to be independently used, the primarycomponent carrier should be set to meet all requirements for the dataand control signaling exchange between a base station and a userequipment.

Meanwhile, the secondary component carrier may include an additionalcomponent carrier that can be activated or deactivated in accordancewith a required size of transceived data. The secondary componentcarrier may be set to be used only in accordance with a specific commandand rule received from a base station. In order to support an additionalbandwidth, the secondary component carrier may be set to be usedtogether with the primary component carrier. Through an activatedcomponent carrier, such a control signal as a UL grant, a DL assignmentand the like can be received by a user equipment from a base station.Through an activated component carrier, such a control signal in UL as achannel quality indicator (CQI), a precoding matrix index (PMI), a rankindicator (RI), a sounding reference signal (SRS) and the like can betransmitted to a base station from a user equipment.

Resource allocation to a user equipment can have a range of a primarycomponent carrier and a plurality of secondary component carriers. In amulti-carrier aggregation mode, based on a system load (i.e.,static/dynamic load balancing), a peak data rate or a service qualityrequirement, a system may be able to allocate secondary componentcarriers to DL and/or UL asymmetrically. In using the carrieraggregation technology, the setting of the component carriers may beprovided to a user equipment by a base station after RRC connectionprocedure. In this case, the RRC connection may mean that a radioresource is allocated to a user equipment based on RRC signalingexchanged between an RRC layer of the user equipment and a network viaSRB. After completion of the RRC connection procedure between the userequipment and the base station, the user equipment may be provided bythe base station with the setting information on the primary componentcarrier and the secondary component carrier. The setting information onthe secondary component carrier may include addition/deletion (oractivation/deactivation) of the secondary component carrier. Therefore,in order to activate a secondary component carrier between a basestation and a user equipment or deactivate a previous secondarycomponent carrier, it may be necessary to perform an exchange of RRCsignaling and MAC control element.

The activation or deactivation of the secondary component carrier may bedetermined by a base station based on a quality of service (QoS), a loadcondition of carrier and other factors. And, the base station may beable to instruct a user equipment of secondary component carrier settingusing a control message including such information as an indication type(activation/deactivation) for DL/UL, a secondary component carrier listand the like.

As stated above, present invention is for random access procedure fordual connectivity environment. Exemplary random access procedure isexplained in detail.

FIG. 7 is a diagram illustrating an operation procedure of a userequipment and a base station during a non-contention based random accessprocedure.

(1) Random Access Preamble Assignment

The non-contention based random access procedure can be performed fortwo cases, i.e., (1) when a handover procedure is performed, and (2)when requested by a command of the base station. Of course, thecontention based random access procedure may also be performed for thetwo cases.

First of all, for non-contention based random access procedure, it isimportant that the user equipment receives a designated random accesspreamble having no possibility of contention from the base station.Examples of a method of receiving a random access preamble include amethod through a handover command and a method through a PDCCH command.A random access preamble is assigned to the user equipment through themethod of receiving a random access preamble (S401).

(2) First Message Transmission

As described above, after receiving a random access preamble designatedonly for the user equipment, the user equipment transmits the preambleto the base station (S402).

(3) Second Message Reception

After the user equipment transmits the random access preamble in stepS402, the base station tries to receive its random access responsewithin a random access response receiving window indicated throughsystem information or handover command (S403). In more detail, therandom access response can be transmitted in the form of a MAC protocoldata unit (MAC PDU), and the MAC PDU can be transferred through aphysical downlink shared channel (PDSCH). Also, it is preferable thatthe user equipment monitors a physical downlink control channel (PDCCH)to appropriately receive information transferred to the PDSCH. Namely,it is preferable that the PDCCH includes information of a user equipmentwhich should receive the PDSCH, frequency and time information of radioresources of the PDSCH, and a transport format of the PDSCH. If the userequipment successfully receives the PDCCH transmitted thereto, the userequipment can appropriately receive a random access response transmittedto the PDSCH in accordance with the information of the PDCCH. The randomaccess response can include a random access preamble identifier (ID)(for example, random access preamble identifier (RA-RNTI)), uplink grantindicating uplink radio resources, a temporary C-RNTI, and timingadvance command (TAC) values.

As described above, the random access preamble identifier is requiredfor the random access response to indicate whether the uplink grant, thetemporary C-RNTI and the TAC values are effective for what userequipment as random access response information for one or more userequipments can be included in one random access response. In this case,it is assumed that the user equipment selects a random access preambleidentifier corresponding to the random access preamble selected in stepS402.

In the non-contention based random access procedure, the user equipmentcan terminate the random access procedure after determining that therandom access procedure has been normally performed by receiving therandom access response information.

FIG. 8 is a diagram illustrating an operation procedure of a userequipment and a base station during a contention based random accessprocedure.

(1) First Message Transmission

First of all, the user equipment randomly selects one random accesspreamble from a set of random access preambles indicated through systeminformation or handover command, and selects a physical RACH (PRACH)resource that can transmit the random access preamble (S501).

(2) Second Message Reception

A method of receiving random access response information is similar tothat of the aforementioned non-contention based random access procedure.Namely, after the user equipment transmits the random access preamble instep S402, the base station tries to receive its random access responsewithin a random access response receiving window indicated throughsystem information or handover command, and receives the PDSCH throughcorresponding random access identifier information (S502). In this case,the base station can receive uplink grant, a temporary C-RNTI, andtiming advance command (TAC) values.

(3) Third Message Transmission

If the user equipment receives s effective random access response, theuser equipment respective processes information included in the randomaccess response. Namely, the user equipment applies TAC and store atemporary C-RNTI. Also, the user equipment transmits data (i.e., thirdmessage) to the base station using UL grant (S503). The third messageshould include a user equipment identifier. This is because that thebase station needs to identify user equipments which perform thecontention based random access procedure, thereby avoiding contentionlater.

Two methods have been discussed to include the user equipment identifierin the third message. In the first method, if the user equipment has aneffective cell identifier previously assigned from a corresponding cellbefore the random access procedure, the user equipment transmits itscell identifier through an uplink transport signal corresponding to theUL grant. On the other hand, if the user equipment does not have aneffective cell identifier previously assigned from a corresponding cellbefore the random access procedure, the user equipment transmits itscell identifier including its unique identifier (for example, S-TMSI orrandom ID). Generally, the unique identifier is longer than the cellidentifier. If the user equipment transmits data corresponding to the ULgrant, the user equipment starts a contention resolution timer.

(4) Fourth Message Reception

After transmitting data including its identifier through UL grantincluded in the random access response, the user equipment waits for acommand of the base station for contention resolution. Namely, the userequipment tries to receive the PDCCH to receive a specific message(504). Two methods have been discussed to receive the PDCCH. Asdescribed above, if the third message is transmitted to correspond tothe UL grant using the user equipment identifier, the user equipmenttries to receive the PDCCH using its cell identifier. If the userequipment identifier is a unique identifier of the user equipment, theuser equipment tries to receive the PDCCH using a temporary cellidentifier included in the random access response. Afterwards, in caseof the first method, if the user equipment receives the PDCCH throughits cell identifier before the contention resolution timer expires, theuser equipment determines that the random access procedure has beenperformed normally, and ends the random access procedure. In case of thesecond method, if the user equipment receives the PDCCH through thetemporary cell identifier before the contention resolution timerexpires, the user equipment identifies data transferred from the PDSCH.If the unique identifier of the user equipment is included in the data,the user equipment determines that the random access procedure has beenperformed normally, and ends the random access procedure.

In LTE Rel-12, a new study on Small Cell Enhancement is started, wherethe dual connectivity is supported. That is, UE is connected to bothMacro cell and Small cell, as shown in FIG. 9.

FIG. 9 is a diagram for explaining dual connectivity of the UE accordingto one aspect of the present invention.

In FIG. 9, the MeNB stands for Macro cell eNB, and SeNB for Small celleNB. Small cell may comprise femto cell, pico cell, etc.

The interface between MeNB and SeNB is called Xn interface. The Xninterface is assumed to be non-ideal; i.e. the delay in Xn interfacecould be up to 60 ms.

The SeNB is responsible for transmitting best effort (BE) type traffic,while the MeNB is responsible for transmitting other types of trafficsuch as VoIP, streaming data, or signaling data. Here, BE type trafficmay be delay tolerable and error intolerable traffic.

To support dual connectivity, various protocol architectures arestudied, and one of potential architectures according to one aspect ofthe present invention is shown in FIG. 10.

FIG. 10 is a diagram showing an exemplary architecture for supportingdual connectivity.

In FIG. 10, MeNB has various radio bearers, signaling radio bearer(SRB), data radio bearer (DRB) and best effort DRB (BE-DRB). In view ofBE-DRB, PDCP and RLC entities are located in different network nodes,i.e. PDCP in MeNB and RLC in SeNB.

In the UE side, the protocol architecture is same as prior art exceptthe MAC entity is setup for each eNB (i.e. M-MAC for MeNB and S-MAC forSeNB). This is because the scheduling nodes are located in differentnodes and two nodes are linked with non-ideal backhaul.

There are two MAC entities in the UE, i.e., M-MAC and S-MAC: The M-MACis responsible for transmission between the UE and the MeNB, and theS-MAC is in charge of transmission between the UE and the SeNB. In thedescription below, M-MAC refers to the M-MAC in the UE and S-MAC refersto the S-MAC in the UE, except the cases when defined otherwise. TheM-MAC in the MeNB and S-MAC in the SeNB will he explicitly specified.

In this dual connectivity situation, since the UE is connected to bothof Macro cell (MeNB) and Small cell (SeNB), the UE needs to perform therandom access (RA) procedure in parallel on the Macro cell and the Smallcell. This may cause a overlapping RA procedures in M-MAC and S-MAC.

In the conventional art, there is only one RA procedure ongoing at anypoint in time based on the UE implementation. However, in Small cellenvironment, it is preferable to make the Macro cell be prioritized overthe Small cell, because the Small cell is used for a best-effort manner.Thus, in one aspect of the present invention, it is proposed for the UEto he able to prioritize the RA procedure on the Macro cell over the RAprocedure on the Small cell during RA procedures.

FIG. 11 is a diagram for explanation on a preferred embodiment of thepresent invention.

As shown in FIG. 11, there are two MAC entities in the UE, i.e., M-MACand S-MAC. The M-MAC is responsible for transmission between the UE andthe MeNB, and the S-MAC is in responsible for transmission between theUE and the SeNB. Accordingly, the M-MAC in the UE performs the RAprocedure on the MeNB and the S-MAC in the UE performs the RA procedureon the SeNB. In the description below, the M-RA refers to the RAprocedure on the Macro cell and the S-RA refers to the RA procedure onthe Small cell. In the description below, it is assumed that the M-MACperforms the M-RA and the S-MAC performs the S-RA.

Also, although it is not shown in FIG. 11, there are two PHY entities inthe UE, i.e., M-PHY and S-PHY. The M-PHY is linked to the M-MAC and theS-PHY is linked to the S-PHY. In the description below, M-PHY refers tothe M-PHY in the UE and S-PHY refers to the S-PHY in the UE.

In this embodiment, it is proposed that, if there are two MAC entitiesin the UE (M-MAC and S-MAC), when the M-MAC starts the M-RA, the M-MACsends an indication of start/completion of M-RA to the S-MAC. When theS-MAC receives the indication of start of M-RA from the M-MAC, the S-MACstops/ignores the initiation of S-RA. When the S-MAC receives theindication of completion of M-RA from the M-MAC, the S-MAC starts thecontention based S-RA if there is any stopped/ignored S-RA.

In the example of FIG. 11, the M-MAC of the UE may receive PDCCH orderrequesting M-RA (S1110). But, the M-MAC of UE may initiate the M-RA byitself.

If the M-MAC is requested an M-RA by a PDCCH order from MeNB or theM-MAC itself, the M-MAC may starts with the M-RA (S1120). When the M-MACstarts with the M-RA, it is proposed that the M-MAC sends an indicationto the S-MAC indicating that the M-MAC starts with the M-RA, which iscalled a ‘start indication’.

In addition, when the M-MAC completes the M-RA, it is proposed thatM-MAC sends an indication of completion of M-RA to the S-MAC, which iscalled a ‘completion indication’ (S1140). The M-MAC may send the‘completion indication’ to the S-MAC regardless of whether the M-RA issuccessfully completed or unsuccessfully completed.

When the S-MAC receives the start indication from the M-MAC, and ifthere is an on-going S-RA, the S-MAC may stop the on-going S-RA. TheS-MAC may discard explicitly signaled preamble for S-RA andra-PRACH-MaskIndex, if any. Also, the S-MAC may flush the HARQ bufferused for transmission of the MAC PDU in the Msg3 buffer.

If the S-MAC is requested an S-RA by the PDCCH order from SeNB (S1130)or by the S-MAC itself after receiving the start indication from theM-MAC, the S-MAC may ignore the request of the S-RA initiation.

The S-MAC may ignore the request of the S-RA initiation during the timeduration between receiving the starting indication of the M-RA from theM-MAC and receiving the completion indication of the S-RA from theM-MAC.

When the S-MAC receives the completion indication from the M-MAC, theS-MAC starts the S-RA as a contention based random access in thefollowing cases.

If there is any stopped S-RA in the S-MAC; or

If there is any ignored S-RA in the S-MAC.

In FIG. 11, after receiving completion indication from M-MAC, when S-MACreceives PDCCH order from SeNB requesting S-RA, the S-MAC initiatesS-RA. (S1150), since it is after receiving the indication indicating thecompletion indication from the M-MAC.

FIG. 12 is a diagram of another example of the present invention.

In FIG. 12, there are two MAC entities in a UE: M-MAC and S-MAC as likeFIG. 11. The UE is configured by the network that when the S-MACreceives a completion indication from the M-MAC, the S-MAC starts thecontention based S-RA if the S-MAC stopped any on-going S-RA uponreception of start indication from the M-MAC.

The MeNB may send a PDCCH order to the UE to request an M-RA (Step 1).When the M-MAC receives the PDCCH order from the MeNB to initialize theM-RA, the M-MAC may start with the M-RA and the M-MAC may send a startindication to the S-MAC (Step 2). The SeNB may send a PDCCH order to theUE to request an S-RA (Step 3). Since the S-MAC had received a startindication from the M-MAC but did not receive a completion indicationfrom the M-MAC yet, the S-MAC may ignore the S-RA request from the SeNB.

When the M-MAC completes the M-RA, the M-MAC may send a completionindication to the S-MAC (Step 4). Since there is no stopped S-RA, theS-MAC does not start the contention based S-RA.

The SeNB may send a PDCCH order to the UE to request an S-RA again (Step5). Since the S-MAC received a completion indication from the M-MAC, theS-MAC may initialize the S-RA.

The MeNB may send a PDCCH order to the UE to request an M-RA again (Step6). Then, the M-MAC starts with the M-RA arid the M-MAC sends a startindication to the S-MAC. When the S-MAC receives a start indication, theS-MAC stops the on-going S-RA.

When the M-MAC completes the M-RA, the M-MAC may send a completionindication to the S-MAC (Step 7). When the S-MAC receives the completionindication from the M-MAC, the S-MAC starts the contention based S-RAbecause the S-MAC stopped the on-going S-RA in step 6

Explanation below is for another aspect of the present invention forprocedures between PHY entity and the MAC entity of the UE.

When the M-RA/S-RA is requested by the PDCCH order from the MeNB/SeNB orby the M-MAC/S-MAC itself, the M-MAC/S-MAC may start with the M-RA/S-RA,respectively. In dual connectivity environment, there can be a situationwhen S-PHY cannot transmit random access preamble due to the coexistenceof the M-RA. It may be because of the lack of transmission power, etc.In the conventional art, this failure of transmission of random accesspreamble is not separately defined. Thus, it has to be managed by therandom access failure. But, this may cause unwanted delay of the randomaccess procedure.

So, in one aspect of the present invention, if two MAC entities in theUE (M-MAC and S-MAC), when the S-PHY is not able to transmit thepreamble for S-RA to the SeNB as instructed by the S-MAC, the S-PHY doesnot transmit the preamble for S-RA to the SeNB. In addition, the S-PHYindicates to the S-MAC that the preamble transmission for S-RA is notperformed. When the S-MAC receives the indication from the S-PHY, theS-MAC continues the S-RA or stops the S-RA. When the S-MAC continues theS-RA upon receiving the indication, the S-MAC does not increase thePREAMBLE_TRANSMISSION_COUNTER or delays the preamble transmission by thebackoff. This scheme is explained in more detail with exemplary figures.

FIG. 13 is a diagram for explaining the procedure according to currentLTE standard considering dual connectivity.

S-MAC of the UE may receive PDCCH order requesting S-RA (S1310). Ofcourse, the S-MAC may initiate S-RA by itself. In this figure, the PDCCHorder is shown as received by S-MAC directly, but actual operation isthat the PDCCH order is received by S-PHY and delivered to S-MAC.

If the S-MAC initiated S-RA, the S-MAC may instruct S-PHY to transmitpreamble to SeNB (S1320). Here, the transmission power of the preambletransmission is set as‘preambleInitialReceivedTargetPower+DELTA_PREAMBLE+(PREAMBLE_TRANSMISSION_COUNTER−1)*powerRampingStep’as explaiend above. Also, S-MAC may instruct the S-PHY to transmit apreamble using the selected PRACH, corresponding RA-RNTI, preamble indexand PREAMBLE_RECEIVED_TARGET_POWER.

As explained above, there can be a situation when S-PHY cannot transmitthe preamble due to the coexistence of the on-going M-RA. It may bebecause of the transmission power limit, but it is not necessarilylimited to this situation. In the current technical standard, there isno procedure for treating this problem. So, it shall be treated asconventional random access failure procedure.

That is, the S-MAC of UE may monitor the PDCCH from SeNB in order toreceive random access response (S1340). This monitoring shall beperformed during the random access monitoring window having a length ofra-ResponseWindowSize subframes.

If the S-MAC fails to receive random access response for its preambletransmission, it determined that the random access procedure is notsuccessful (S1350). Specifically, if no Random Access Response isreceived within the RA Response window, or if none of all receivedRandom Access Responses contains a Random Access Preamble identifiercorresponding to the transmitted Random Access Preamble, the RandomAccess Response reception is considered not successful and S-MAC mayperform procedures for random access failure (S1360). Procedures forrandom access failure includes the following:

The S-MAC increments PREAMBLE_TRANSMISSION_COUNTER by 1.

if PREAMBLE_TRANSMISSION_COUNTER=preambleTransMax+1, that is, if thePREAMBLE_TRANSMISSION_COUNTER reaches to the maximum alloable number, itindicate a Random Access problem to upper layers (if the Random AccessPreamble is transmitted on the PCell, or consider the Random Accessprocedure unsuccessfully completed (if the Random Access Preamble istransmitted on an SCell).

If in this Random Access procedure, the Random Access Preamble wasselected by MAC: based on the backoff parameter in the UE, select arandom backoff time according to a uniform distribution between 0 andthe Backoff Parameter Value; delay the subsequent Random Accesstransmission by the backoff time; and proceed to the selection of aRandom Access Resource from the first.

Those skilled in the art would realize that the above procedure isinefficient, so the following schemes are proposed.

FIG. 14 is a diagram for explaining one example of the presentinvention.

If the M-MAC is requested an M-RA by a PDCCH order or the M-MAC itself,the M-MAC starts with the M-RA. If the S-MAC is requested an S-RA by aPDCCH order or the S-MAC itself, the S-MAC starts with the S-RA (S1310).

When the M-MAC starts with the M-RA, the M-MAC instructs the M-PHY totransmit the preamble for M-RA using the selected PRACH and thePREAMBLE_RECEIVED_TARGET_POWER. When the S-MAC starts with the S-RA, theS-MAC instructs the S-PHY to transmit the preamble for S-RA using theselected PRACH and the PREAMBLE_RECEIVED_TARGET_POWER (S1320).

When S-PHY receives the instruction from the S-MAC to transmit thepreamble for S-RA, if the S-PHY is not able to transmit the preamble forS-RA to the SeNB using the PRACH, or the PREAMBLE_RECEIVED_TARGET_POWERas instructed from the S-MAC, the S-PHY does not transmit the preamblefor S-RA to the SeNB (S1330). This may be cause by on-going M-RA asshown in FIG. 14.

When the S-PHY does not transmit the preamble for S-RA to the SeNB, itis proposed that the S-PRY sends an indication to the S-MAC whichindicates that S-PHY is not able to transmit the preamble for S-RA tothe SeNB (S1410).

When the S-MAC receives this indication from S-PHY, the S-MAC maycontinue or stop S-RA without performing procedures for the randomaccess failure (S1420). That is, S-MAC does not have to perform theprocedures for the random access failure as explained with regards toFIG. 13. If the S-MAC determines to continue the S-RA, S-MAC mayreselect the random access resource and instruct again to S-PHY totransmit the preamble after predetermined backoff time. If the S-MACdetermines to stop the S-RA, S-MAC may discard the random accessresource and flush the HARQ buffer for random access procedure.

FIGS. 15 and 16 are detailed examples for the scheme explained withregards to FIG. 14.

Procedures before S1410 are the same as FIG. 14 both for FIGS. 15 and16. FIG. 15 is for procedures when S-MAC determines to continue S-RA.FIG. 16 is for procedures when S-MAC determines to stop S-RA.

Referring to FIG. 15, when the S-MAC receives indication from S-PHYindicating that the preamble transmission is impossible, the S-MAC maydetermine to continue S-RA. Specifically, the S-MAC may proceed to theselection of a Random Access Resource without increasing thePREAMBLE_TRANSMISSION_COUNTER (S1420-1). In this case, as thePREAMBLE_TRANSMISSION_COUNTER is not increased, thePREAMBLE_RECEIVED_TARGET_POWER is not increased.

The S-MAC may proceed to the selection of a Random Access Resource bysetting the backoff parameter value in the UE. The backoff parametervalue can be pre-defined between the UE and the network or configured bythe network using an RRC/MAC/PHY signal. Based on the backoff parametervalue in the UE, the S-MAC may select a random backoff time according toa uniform distribution between 0 and the backoff parameter value. TheS-MAC delays the subsequent Random Access transmission by the backofftime, and then proceeds to the selection of a Random Access Resource.

Then, the S-MAC may instruct S-PHY to transmit preamble (S1420-2)

Referring to FIG. 16, when S-MAC receives indication from S-PHYindicating that the preamble transmission is impossible, the S-MAC maydetermine to stop S-RA. Specifically, the S-MAC may discard explicitlysignaled preamble for S-RA and ra-PRACH-MaskIndex, if any (S1420-1).Further, the S-MAC may flush the HARQ buffer used for transmission ofthe MAC PDU in the Msg3 buffer (S1420-2). By these actions, the S-MACmay stop the on-going S-RA.

If the S-MAC receives the completion indication indicating that theM-MAC completes the M-RA, if the S-MAC stopped the S-RA upon receivingan indication from the S-PHY indicating that the S-PHY is not able totransmit the preamble for S-RA, the S-MAC may start the contention basedS-RA.

In this case, the S-MAC may start the contention based S-RA regardlessof whether the S-MAC stopped the S-RA which is initiated by the PDCCHorder from the SeNB or by the S-MAC itself.

FIG. 17 is a block diagram of a communication apparatus according to anembodiment of the present invention.

The apparatus shown in FIG. 17 can be a user equipment (UE) and/or eNBadapted to perform the above mechanism, but it can be any apparatus forperforming the same operation.

As shown in FIG. 17, the apparatus may comprises a DSP/microprocessor(110) and RF module (transmiceiver; 135). The DSP/microprocessor (110)is electrically connected with the transciver (135) and controls it. Theapparatus may further include power management module (105), battery(155), display (115), keypad (120), SIM card (125), memory device (130),speaker (145) and input device (150), based on its implementation anddesigner's choice.

Specifically, FIG. 17 may represent a UE comprising a receiver (135)configured to receive signal from SeNB/MeNB, and a transmitter (135)configured to transmit signals to the network. These receiver and thetransmitter can constitute the transceiver (135). The UE furthercomprises a processor (110) connected to the transceiver (135: receiverand transmitter).

Also, FIG. 17 may represent a network apparatus comprising a transmitter(135) configured to transmit signals to a UE and a receiver (135)configured to receive signal from the UE. These transmitter and receivermay constitute the transceiver (135). The network further comprises aprocessor (110) connected to the transmitter and the receiver.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

The embodiments of the present invention described herein below arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment.

It is obvious to those skilled in the art that claims that are notexplicitly cited in each other in the appended claims may be presentedin combination as an embodiment of the present invention or included asa new claim by subsequent amendment after the application is tiled.

In the embodiments of the present invention, a specific operationdescribed as performed by the BS may be performed by an upper node ofthe BS. Namely, it is apparent that, in a network comprised of aplurality of network nodes including a BS, various operations performedfor communication with an MS may be performed by the BS, or networknodes other than the BS. The term ‘eNB’ may be replaced with the term‘fixed station’, ‘Node B’, ‘Base Station (BS)’, ‘access point’, etc.

The above-described embodiments may be implemented by various means, forexample, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the above-described method has been described centering on anexample applied to the 3GPP LTE system, the present invention isapplicable to a variety of wireless communication systems, e.g. IEEEsystem, in addition to the 3GPP LTE system.

What is claimed is:
 1. A method for control a transmission power of arandom access preamble transmission, the method comprising: determiningwhether the random access preamble transmission is an initialtransmission or a retransmission; determining whether there is a noticefor suspending a power ramping; incrementing a counter value for thepower ramping, based on determining that the random access preambletransmission being the retransmission, and based on determining thatthere is no notice for suspending the power ramping, wherein the countervalue is not incremented based on determining that there is the noticefor suspending the power ramping; and performing the random accesspreamble transmission with the transmission power determined based onthe counter value.
 2. The method of claim 1, wherein the notice forsuspending the power ramping is issued based on whether there is a dropof a random access preamble due to a power allocation in a dualconnectivity (DC) operation.
 3. The method of claim 2, wherein the dropof the random access preamble due to the power allocation in the DCoperation happens based on an overlap between
 4. The method of claim 1,wherein the transmission power is determined based on the counter valuemultiplied by a preamble power ramping step.
 5. A user equipment (UE)for control a transmission power of a random access preambletransmission, the UE comprising: a processor configured to determinewhether the random access preamble transmission is an initialtransmission or a retransmission, and whether there is a notice forsuspending a power ramping, wherein the processor increments a countervalue for the power ramping, based on determining that the random accesspreamble transmission being the retransmission, and based on determiningthat there is no notice for suspending the power ramping, wherein theprocessor does not increment the counter value based on determining thatthere is the notice for suspending the power ramping; and a transceiverconnected to the processor and configured to perform the random accesspreamble transmission with the transmission power determined by theprocessor based on the counter value.
 6. The UE of claim 5, wherein theprocessor issues the notice for suspending the power ramping based onwhether there is a drop of a random access preamble due to a powerallocation in a dual connectivity (DC) operation.
 7. The UE of claim 6,wherein the drop of the random access preamble due to the powerallocation in the DC operation happens based on an overlap between 8.The UE of claim 5, wherein the processor determines the transmissionpower based on the counter value multiplied by a preamble power rampingstep.